Light emitting diodes, or LEDs, are being used in an ever-growing number of ways, and the size of LEDs is becoming smaller to accommodate larger numbers of diodes in less space. A diode is the simplest sort of semiconductor device and includes an arrangement of an electron rich (N-type) material in electrical contact with a hole rich (P-type) material. When current flows across a diode, negative electrons move one way and positive holes move the other way. When an electron encounters and ‘falls into’ a hole it loses energy, and the energy is emitted as a light photon. LEDs output more lumens of light per-watt than regular incandescent bulbs, and some LEDs may have a lifetime of 50,000 hours or more.
Nitride semi-conductors are a desirable semiconductor material system for light-emitting devices operating in the green-blue-ultraviolet spectrum. Presently, nitride semiconductor structures grown on sapphire substrates are used for conventional blue LED, green LED, ultraviolet (UV) LED, and blue laser diode (LD) devices. These devices may be usable in a variety of applications including full-color displays, traffic lights, image scanners, solid state lighting and high-density optical storage disks. However, nitride semiconductors are difficult and costly to produce as bulk single crystals. For this reason, hetero-epitaxial technology is often used to grow nitride semiconductors on different material substrates such as sapphire. In order to improve the crystalline quality of the grown layers, buffer layer growth at low temperature, patterning, epitaxial lateral overgrowth, or additional growth steps may be required to reduce crystal defects to levels necessary for operation of light-emitting devices. Further improvements in crystalline quality are needed to enable development of smaller light-emitting devices with longer life time, higher output power, and lower cost relative to conventional devices.
Because sapphire has a low thermal conductivity and is electrically insulating, the functionality of nitride semiconductor structures on sapphire is limited. Both electrical contacts of a light-emitting device grown on a sapphire substrate are located on the top surface to form a lateral type device. This reduces the usable area of light-emission. Because both contacts are located on the top surface in a lateral device, significant lateral current flows through the chip resulting in heating of the light-emitting device which accelerates the degradation of the device. In addition, the coefficient of thermal expansion of sapphire is also poorly matched to nitrides and its alloys. As a result, the growth of nitride-based films on sapphire substrates presents challenges that scale with wafer diameter. Because of these challenges, manufacturers have found it difficult to provide larger substrate sizes despite the potential for associated cost reductions.
It therefore remains desirable to provide alternative substrates for semiconductor devices, and to reduce the cost of semiconductor diodes, such as LEDs, while decreasing the size of the diodes and producing more diodes per unit space.